The present invention relates to a magnetic resonance imaging apparatus (hereafter referred to as an MRI apparatus), and more particularly to a large-scale MRI apparatus having magnetostatic-field-generating magnets for generating strong fields and a magnetic shield for insulating the high fields.
The MRI apparatus to obtain tomographic images of a human body by using the nuclear magnetic resonance phenomenon are widely used in medical institutions. In an examination using the MRI apparatus, magnets are required which generate uniform field strength in a space, where an examined region of an examinee is placed, to produce images reflecting the internal structure of the examined region.
For the magnets of the MRI apparatus, permanent magnets, normal conducting magnets and superconducting magnets have been put into practical use. The superconducting magnets, which can achieve higher magnetostatic field strength, are finding wider applciations than permanent magnets and normal conducting magnets. With MRI apparatus using superconducting magnets capable of providing uniform and high magnetostatic field strength, it has become possible to obtain high-quality images also in examinations by various methods of high-speed photography.
As for types of magnet, long, cylindrical solenoid type magnets have been used. On the other hand, various kinds of the open type magnet devices, which are open at the lateral sides and the front side thereof, have been developed, to get rid of the examinee""s feeling or fear of being confined in a narrow space when entering the space between the magnet devices, and also to make it possible to perform interventional operations under the MRI examination as means of monitoring in medical treatment or surgery. Also in the MRI apparatus having such an open type magnet devices, high field magnet devices using superconducting magnets have been put into practical application to realize high-speed high-quality picture taking. The open type MRI apparatus is disclosed in JP-A-11-197132, JP-A-11-155831 and JP-A-10-179546, for example.
However, as high magnetostatic field strengths have been achieved in the examination space where an examinee is placed, a problem has arisen that the magnetic flux density around the magnet device increases. This problem is conspicuous in the open type MRI apparatus. The magnetic flux density outside the magnet is called leakage field strength and defined by a distance from the magnet center to a location where a flux density of 0.5 mT is measured, and this distance is normally desired to be equal to or less than the size of a room where the MRI apparatus (magnet device) is installed. However, in an open-type MRI apparatus, such as mentioned above, which has high field superconducting magnets of an examination magnetic field density of 0.7 to 1.0 T, this distance is as large as more than 10 m.
To minimize the leakage field to the outside of the examination room, a possible solution is to enclose the walls of the examination room, where the magnets are installed, by a ferromagnetic material. However, to confine the superconducting magnets"" leakage field strength of 0.7 to 1.0T within the range of the examination room of an ordinary size (8xcx9c10 m for example), it is necessary to put up a magnetic shield of not less than 10 cm in thickness, which is not practical.
Another solution may be considered to reinforce the magnetic shield by combining the magnets with a magnetic circuit composed of a ferromagnetic material. However, when a ferromagnetic substance is located close to the superconducting coils, there is a possibility that the ferromagnetic substance directly affects the distribution of the flux density in the examination space, and disturbs the field uniformity. If the field uniformity is disturbed, it becomes impossible to perform those imaging methods that require field uniformity of higher mode. For example, in the fat signal suppression method, by using a difference in resonance frequency of about 3xcx9c4 ppm between a water signal and a fat signal, a high luminance signal generated by the fat tissue of the examined region of a human body is suppressed, but this method is not applicable if a higher mode on the z axis occurs as much as 30 ppm.
It is known that attempts to correct a non-uniform field of higher mode by using current shims or miniscule iron pieces are often turned out to be impractical because high current shims or a large number of iron pieces are required for this purpose. Therefore, in the design stage of the magnet devices used in the conventional MRI apparatus, the layout of superconducting coils is decided so as to prevent the occurrence of a non-uniform field of higher mode. However, if one tries to realize superconducting magnets just as designed in the form of open-type superconducting magnets using a large-scale magnetic circuit (iron yoke), there are problems, such as an increased complexity of the structure of the iron yoke, the extreme complexity of processing materials to make the yoke in a monolithic form, and increased waste of materials. Yet another problem is that with the magnet devices using a monolithic iron yoke, it becomes very difficult to mount, inspect and maintain the superconducting coils.
The weight of the MRI apparatus on completion is often over 10 tons. In such a case, many workers and heavy machinery are required to transport the MRI apparatus to the site and install it. Particularly, in the case of MRI apparatus that generates a high magnetic field by superconducting magnets, because its gross weight is several tens of tons, it should preferably be designed by taking the ease of transport and installation at the site into consideration. At the same time, it is required that the MRI apparatus is so structured as to keep a uniform field throughout the examination space.
An object of the present invention is to provide an MRI apparatus having a high magnetic field magnet, as well as an assembly method of the same, which is capable of efficiently confining a leakage magnetic field within a space in the room where the apparatus is installed, particularly with an open type MRI apparatus. Another object of the present invention is to provide an open type strong magnetic field MRI apparatus, as well as an assembly method of the same, which is capable of maintaining a high uniformity of magnetic field in an examination space and which makes it possible to apply examination methods, such as a fat signal suppression method. Yet another object of the present invention is to provide MRI apparatus, as well as an assembly method of the same, which makes it easy to assemble, inspect and maintain the superconducting coils.
According to an aspect of the present invention, a magnetic resonance imaging apparatus (hereafter referred to as MRI apparatus) comprises a pair of a first magnet device and a second magnet device for generating a magnetostatic field, the pair of magnet devices being installed in face-to-face relation with each other across an examination space for accommodating an examinee; a gradient-field generating system, a high-frequency (RF) field generating system; and a yoke for combining the first and second magnetic devices to guide magnetic fluxes generated by the first and second magnetic devices to thereby form a closed magnetic circuit, wherein the yoke includes a first plate member fixed to a first magnet device, a second plate member fixed to a second magnet device and one or more support-post members interconnecting the first plate member and the second plate member, each of the first, second and support-post members includes a plurality of segments formed in such a shape as to minimize the leakage field strength from the first magnet device and the second magnet device.
According to an embodiment of the MRI apparatus of the present invention, the yoke may include a plurality of segments which have different shapes, and those segments of different shapes may be combined in such a pattern as to match the magnetic induction lines of the fluxes generated by the magnetostatic field generating magnets.
According to an aspect of the MRI apparatus of the present invention, the yoke of the magnetic circuit is formed by a combination of a plurality of segments as subdivisions thereof, the segments of different sizes can be easily manufactured by an ordinary cutting process or a machining process of standardized steel plate product available on the market, and easily transferred by a simple carriage to the installation site of the MRI apparatus, and assembled to build the MRI apparatus there. If the yoke is composed of small segments and the respective segments are formed in appropriately selected shapes, a yoke of an optimum shape can be obtained which is suitable for the flux distribution in the yoke and which minimizes the leakage magnetic field. It is not practical to try to obtain a yoke of such an optimum shape that matches the flux distribution by cutting or by any other machining process from a steel material of large bulk.
According to further aspect of the MRI apparatus of the present invention, a method of assembling the MRI apparatus is carried out as follows. In a desired place, a plurality of segments of the first plate member are stacked by fixing one after another, and those segments are connected together to thereby assemble the first plate member. Then, a plurality of segments of the support-post members are fixed one after another to the first plate member, and the support-post segments are connected to thereby assemble the support-post members. After this, a magnet device assembly is provided which connects the first magnet device and the second magnet device together by a connection pipe, the first and second magnet devices being arranged in face-to-face relation with each other across a predetermined space. The magnet device assembly is fixed to the first plate member and also to the support-post member in such a way that the first magnet device is located on the first plate member. Then, a plurality of segments of the second plate member are fixed one after another on the second magnet device, and thus the second plate member is assembled.
According to an aspect of assembling the MRI apparatus of the present invention, the MRI apparatus may be configured such that the magnet devices are to be mounted in the course of assembly of the magnetic shield, in which case, the magnet devices can be assembled, inspected and maintained merely by dismounting at least a part of the yoke.